Grinding of rubber is a difficult application, due to the build up of rubber on the face of the grinding wheel. The friction generated by the rubber-on-rubber contact, as the wheel cuts, combined with the poor thermal conductivity of rubber, leads to excessive heat generation. This, in turn, leads to wheel cracking and sometimes-even breakage. This problem manifests itself most acutely when vitrified wheels are used to grind soft rubbers. For this reason, the abrasive industry has often been hesitant to supply grinding wheels for this application.

1. ROLL GRINDING OVERVIEW

Grinding of paper mill calender rolls is one of the most difficult grinding operations in industry. They are very long relative to diameter, hard, and have close tolerances. They require a good to excellent surface finish. Grinding of other types such as stainless sucker rolls also is no easy task.

Historically, paper mill rolls have been time consuming to rough and more so to finish. Considerable operator skill has been required as well as attention to the machine and surroundings.

Difficulties encountered may include roll deflection and sag, bearing induced effects, vibration, roll to wheel harmonics, wheel glazing, stainless steel loading and the like. Grinding, especially finish grinding, can be a sensitive process. One roll might grind with few problems. The next could take twice as long.

2. ROUGHING AND FINISHING PHASES

The choice of grinding wheel for roughing and/or finishing has always been of prime importance. A wheel that is too hard, soft, has hard spots or varies wheel to wheel can cause much time loss. The loss may be due to the operator having to compensate by:

slowing travel across the roll

reducing depth of cut (more passes)

regrinding to remove chatter, etc.

dressing wheel frequently

Subtle inefficiencies can occur in roll grinding. Time-held practice may seem satisfactory. For example, the wheel traditionally used may seem to be as aggressive as can be expected. Its abrasive action, however, could be considerably less than with another type wheel. Technology has advanced.

Additionally, roughing and finishing with one wheel may take more time than with the best wheel for each. Particularly with chilled iron rolls, a wheel hard enough for roughing can be too hard for ease of finishing. The problem appears as chatter, spiral bands, heat buildup, grain pits and other such defects. Trying to finish these out requires more passes and at a slow pace.

3. TRANSITION PHASE

Also to be considered when roughing and finishing rolls with one wheel is overlap between the two phases. Typically, as a roll approaches the roughing depth required, gradual adjustments are made to wheel feeds and speeds. This transition, in effect, is preparation for finishing. It is necessary but has the disadvantage of lengthening both roughing and finishing phases.

More efficient would be to reduce the transition phase. This would be possible if using a wheel better designed for finishing. The finishing wheel must be tough enough to remove grinding marks but resilient enough not to impart new ones. With this system, a very aggressive wheel would fast rough, a very smooth wheel would fast finish and little transition phase would be necessary.

4. ROUGHING WHEEL OVERVIEW

Paper mill roll grinding wheels consist mainly of abrasive grains and a resin bond for holding the grains together. The abrasive generally used is silicon carbide. This black grain is sharp, hard and well suited for most rolls.

The bond generally used in past decades has been shellac resin. A property of shellac resin is that it becomes somewhat soft during the heat of grinding. This makes the wheel more able to release dulled abrasive grains and expose sharp new ones. A roll wheel must wear or breakdown at the appropriate rate in order to grind effectively.

Hard Rubbers

Much depends on the hardness of the rubber to be ground. In general, the harder the rubber to be ground the easier it is. So for example an ebonite can be ground relatively easily with a conventionally structured vitrified wheel, although it is not recommended due to the thermal shock created whilst grinding is taking place.

For other hard rubbers e.g. 80-90 shore A, a soft resinoid bonded wheel should be used, this resin bond type grinding wheel is the only product any abrasives supplier should recommend when grinding hard rubbers. An example of such a grinding wheel is as follows;

Wheel grading: C46-K7-B

Explanation of the coding used,

C = Silicon Carbide.

46 = Grit size.

K = Hardness of wheel.

7 = Porosity. The higher the number the larger the porosity.

B = Resinoid Bonded.

Note, the above example of coding is purely for demonstration purposes.

The above coding would be a good starting point for the grinding of hard rubbers.

When grinding rubber rollers it is difficult to determine an exact specification for each rubber type and its varying hardness's, therefore the operator and supplier have to gain experience of each wheel and the characteristics it brings.

Once these experiences are made, only then can they proceed to try something a little different, i.e. they could possibly change the code to a C60-K7-B thereby, possibly creating a better finish on the rubber surface due to the density of the wheel being increased. Although the operator will see an improvement in surface finish it is also likely that the wheel would not be able to grind to finish size as quick due to the change in grit size.

This is why each operator and supplier will have and keep their own preferences. Note I have mentioned the supplier when talking and assessing the grinding wheel, this is an extremely important consideration when developing the correct grades of wheels. Suppliers of grinding wheels have very little knowledge of grinding rubber rollers; they should be involved in every modification and change you make in order to reach optimization.

Soft Rubbers

The very soft rubbers commonly used in the offset printing industry present the greatest difficulty to the abrasive supplier.

These materials are inherently tacky and extremely elastic, leading them to clog the face of the grinding wheel very rapidly, leading to excessive heat generation and the associated problems.

The most effective and safest way to grind these materials is with a resin-bonded bubble alumina wheel, not a vitrified wheel. The very high porosity of this wheel type allows the wheel to continue cutting, without clogging the face. The porosity also allows heat dissipation, reducing the incidence of cracking and breakage.

The following specifications for grinding this type of soft rubbers are:

Roughing: WBA24-K-BB

Finishing: WBA46-K-BB

Note the difference in coding, again, the 24 is representing a more open structure thus giving the wheel a greater ability to take larger cuts from the roller being ground, whereas the 46 will give the roller surface a better finish due to the structure of the wheel being closer together.

The following grades may be used when considering the purchase of such a grinding wheel, 24, 36, 46, 60, 80 - 24 being the roughest and 80 the smoothest.

Again each operator and supplier will build up there own preferences that they prefer to use and employ.

Speed of grinding

One major point to consider when purchasing vitrified or resin bonded grinding wheels is speed. The maximum recommended surface speed of any vitrified or resin bonded wheel is 35 meters per second this should never be exceeded. Please view our Speed Conversion Table for further details.

All suppliers of abrasives should be members of F.E.P.A. this is the Federation of European Producers of Abrasives.